Free-radical reactions of halogenated bridged polycyclic compounds. Part XIV. The cobalt(II) chloride-catalysed reaction of methylmagnesium iodide with 5-endo-bromo-1,2,3,4,7,7-hexachloronorborn-2-ene and related compounds

Abstract

The cobalt(II) chloride-catalysed reaction of methylmagnesium iodide with 5-endo-bromo-1,2,3,4,7,7-hexachloronorborn-2-ene affords 5-endo-bromo-1,2,3,4,7-syn-pentachloronorborn-2-ene, 5-endo-bromo-1,2,3,4,7-anti-pentachloronorborn-2-ene, 1,2,3,5-endo, 6-endo-pentachloronorborn-2-ene, 1,2,3,5,6-pentachloronorborna-2,5-diene, 1,2,3,5-tetrachloronorborna-2,5-diene, and 1,2,3-trichloro-5-methylnorborna-2,5-diene, together with traces of 1,2,3,5-endo,6-exo-pentachloronorborn-2-ene and 1,2,3,5-endo-tetrachloronorborn-2-ene. Product formation may be largely rationalised in terms of the intermediate formation of free radicals produced by removal from the starting halide of the 5-endo-bromine and bridge chlorine atoms. The reactions of 5-endo-bromo-1,2,3,4,7-syn-pentachloronorborn-2-ene, 5-exo-bromo-1,2,3,4,7-syn-pentachloronorborn-2-ene, 5-endo-bromo-1,2,3,4,7-anti-pentachloronorborn-2-ene, 5-endo-bromo-1,2,3,4-tetrachloronorborn-2-ene, and 5-exo-bromo-1,2,3,4-tetrachloronorborn-2-ene are similar. The reaction of 1,2,3,5,6-pentachloronorborna-2,5-diene is unusual in that it results in reduction of a double bond to form 1,2,3,5-endo,6-endo-pentachloronorborn-2-ene. In contrast, the reaction of 1,2,3,5-tetrachloronorborna-2,5-diene results in substitution of a vinylic chlorine atom by a methyl group to give 1,2,3-trichloro-5-methylnorborna-2,5-diene in a reaction which may involve vinyl-type radical intermediates.